1. Field of The Invention
The present invention relates generally to chemical vapor deposition (CVD) of thin films and, more particularly, to specific source compounds useful in the CVD of ferroelectric thin films.
2. The Prior Art
Lead zirconate titanate (PZT) ferroelectric thin films and their related alloys, such as ferroelectric PLZT thin films (i.e., less than ten microns and preferably about 0.15 micron to about one micron) find widespread applications in capacitors, specifically high energy density capacitors (requiring a dielectric possessing a high dielectric constant), for fast, nonvolatile integrated digital memories, for pyroelectric infrared sensors, for optical switches with nonlinear optical properties, for piezoelectric transducers and for surface acoustic wave devices. Despite a heavy demand, utilization of such ferroelectric thin films has been hampered by the non-existence of a reliable, large scale manufacturing process for economically producing device-quality ferroelectric thin films. To be sure, this was not for a lack of trying.
Beginning in the 1970's and continuing to the present, a lot of investigative effort has been directed at depositing ferroelectric thin films by sputtering methods, see M. Ishida et al., "Preparation and properties of ferroelectric PLZT thin films by rf sputtering," Journal of Applied Physics, Vol 48, No. 3, March 1977, pp. 951-953, and R. Takayama et al., "Preparation of epitaxial Pb(Zr.sub.x Ti.sub.1-x)O.sub.3 thin films and their crystallographic, pyroelectric, and ferroelectric properties," J. Appl. Physics 65(4), 15 Feb. 1989, pp. 1666-1670. A further effort involving the preparation of ferroelectric thin films has focused on sol gel processing, see G. Yi et al., "Preparation of Pb(Zr,Ti)O.sub.3 thin films by sol gel processing: Electrical, optical, and electro-optic properties," J. Appl. Physics 64(5), 1 Sept. 1988, pp. 2717-2724. Both of these processes suffer from: limitations in achieving uniformity and reproducibility (including mixed phases of microscopically varying compositions) in the thin films produced, relatively low production rates, difficulties encountered by incompatibility of substrates and, inability to adapt the process to scale-up production. Additionally, the thin films produced have exhibited unacceptable levels of impurity concentrations, i.e., in excess of 100 parts per million atomic concentration (ppma) of contaminants.
With the advent of chemical vapor deposition (CVD) and metal-organic chemical vapor deposition (MOCVD), several workers in the field have begun to use such techniques with several materials and with various degrees of success. As known, CVD offers fast deposition rates, CVD reactors can be scaled up to the deposition on large-area substrates and film uniformity is excellent with the CVD process. Most of this effort has, however, focused on the CVD or MOCVD deposition of ferroelectric lead-titanate (PbTiO.sub.3) thin films. See S. Yoon et al., "Preparation and Deposition Mechanism of Ferroelectric PbTiO.sub.3 Thin Films by Chemical Vapor Deposition," J. Electrochem. Soc. (Dec. 1988), pp. 3137-3140; B. S. Kwak et al; "Metalorganic chemical vapor deposition of PbTiO.sub.3 thin films," Appl. Phys. Lett. 53(18) 31 Oct. 1988, pp. 1702-1704; and S. L. Swartz et al., "Characterization of MOCVD PbTiO.sub.3 Thin Films," presented at the Zurich ISAF Conference, Sept, 1988.